Dear Ning Wang,
The ability to actively control trailing-edge flaps has never been a fully supported feature of FAST / OpenFAST. There has been discussion on this forum regarding using existing features of FAST plus customized enhancements to model actively controlled trailing-edge flaps, e.g. see: Movable trailing edge flaps implementation - #11 by Jason.Jonkman. Moreover, a recent pull request into OpenFAST (not yet merged into OpenFAST dev) should allow this capability to be fully supported in a future version, see: github.com/OpenFAST/openfast/pull/213.
Best regards,